1. Field of the Invention
The present invention relates to highly densified silicon nitride sintered bodies having high mechanical strength and fracture toughness and a method for producing the same.
2. Description of the Prior Art
Silicon nitride sintered bodies have excellent mechanical strength, fracture toughness, heat resistance, thermal shock resistance and corrosion resistance and therefore, they have been extensively developed as heat resistant high strength materials.
Silicon nitride is a substance having high covalency and itself has very poor sinterability, so that in order to obtain sintered bodies having a high density, it has been known that a sintering aid, such as MgO, Al.sub.2 O.sub.3, Y.sub.2 O.sub.3, ZrO.sub.2 and the like is generally added. That is, as already known silicon nitride sintered bodies, ones containing MgO as a sintering aid, ones containing MgO and ZrO.sub.2, or ones containing Y.sub.2 O.sub.3, Al.sub.2 O.sub.3 and MgO have been known.
However, the prior silicon nitride sintered bodies have not always satisfied mechanical strength requirements, the fracture toughness requirements and the like. Namely, prior sintered bodies produced through a pressureless sintering process have had a four point flexural strength at room temperature of lower than 70 kg/mm.sup.2 and a fracture toughness of lower than 6 MN/m.sup.3/2 and sintered bodies having improved properties have been required as high strength materials.
As the sintering process for obtaining silicon nitride sintered bodies having high density, a pressureless sintering process and a sintering process under high pressure have been known.
The sintering process under high pressure positively applies mechanical pressure for densifying the sintered body and can provide articles having high density, but it is difficult to produce articles having complicated shapes. Further the productivity is poor and the firing installation is complicated. This results in higher production costs.
On the other hand, the pressureless sintering process can produce more easily and cheaply articles having complicated shapes than the sintering process under high pressure and therefore is high in commercial value, but for the densification, it is necessary to make the grain size of the raw material powder more fine; to increase an amount of the sintering aid; and to raise the firing temperature. However it is impossible to obtain articles having such high density and strength as in the sintered bodies obtained in the high pressure process.